[4] However, on sites with little water or intense sun it will only attain shrub height, and even those that reach tree size will more typically be 4.6–6.1 metres (15–20 feet) tall in open juniper woodlands.

[5] Younger trees have a narrow pyramidal shape, but develop into a rounded, oval, or spreading and irregular crown when older.

Its proper classification has continued to be debated by botanists with Per Axel Rydberg proposing to move it to his new genus as Sabina scopulorum in 1900 and Albert Edward Murray publishing a paper in 1983 that reclassified it as a subspecies under the name Juniperus virginiana subsp.

[21] In one unusual locality in Spring Valley, Nevada they are known as "swamp cedar" for growing in a relatively wet canyon bottom.

[26] In casual conversation the trees will usually simply be called "cedars" or "junipers" without qualification by residents of the western United States.

[19] The species is native to western North America, in Canada in south British Columbia and southwest Alberta, in the United States sporadically from Washington east to North Dakota, south to Arizona and also locally western Texas, and northernmost Mexico from Sonora east to Coahuila.

Though obtaining a greater size in more sheltered locations they will successfully grow on rock outcrops with no soil and on high ridges.

[8] In the mountains to the north of Colorado and Utah the trees grow on relatively dry sites, often south facing slopes.

[31] In one instance it has adapted to quite extreme conditions for a juniper, growing on wet clay soils in Spring Valley, Nevada.

[31] Towards the end of the Last Glacial Period, from 13,500 to 10,000 years before the present, Rocky Mountain juniper grew at much lower elevations in what is now the great basin and desert Southwest.

Evidence from pack rat middens show that plant vegetation zones were 300 to 1100 meters lower in elevation than they are at present.

During the ice age the north of its present range was largely covered in glaciars and far too cold for it in areas not covered in ice with populations only reaching as far north as present day south-eastern Wyoming, southeastern Oregon, southern Idaho, and northern Colorado insolated refuges.

[8] At higher elevations of the Intermountain West dominated by Douglas fir (Pseudotsuga menziesii), it may be considered more of a pioneer species.

[32] Rocky Mountain juniper is a relatively slow growing species with an average age (at one site) of eight years for saplings 30 centimeters in height.

[5] Until they are approximately 50 years old Rocky Mountain junipers are vulnerable to fire due to thin bark and relatively large concentrations of resins and oils.

[17] Historically fire was one of the factors that maintained open, grassy plains and prevented the invasion of trees like Rocky Mountain juniper.

[12] Prior to European-American settlement of the west, fires typically reoccurred at intervals of 50–100 years in most forests including piñon-juniper woodlands.

[34] Studies of their winter foraging habits show that together with big sagebrush (Artemisia tridentata) and bitterbrush (Purshia spp.)

[15] The presence of cover in the form of small trees and large bushes, like Rocky Mountain Juniper, is also important to mule deer.

[5] Two species of mites are known to use Rocky Mountain juniper leaf-scales as a food source, Oligonychus ununguis and Eurytetranychus admes.

[38] In Montana a study of pine-juniper woodlands with Rocky Mountain junipers found that mourning doves will make use of them as a nesting site, though they prefer limber pines.

A different study of piñon-juniper woodlands found that mourning doves prefer junipers as nesting locations.

[39] On the northern plains Rocky Mountain juniper stands support a wide variety of bird species, directly or indirectly.

Other birds observed year round in the groves include black-capped chickadees, black-billed magpies, and long-eared owls.

[41] Many songbirds enthusiastically eat the soft, slightly sweet cones including American robins, solitaires, and waxwings.

Due to a combination of chemical inhibitors and a waterproof coating on the seeds they only germinate at high numbers in their second year off the tree.

[52] Like most varieties derived from Rocky Mountain juniper it is intolerant of hot, humid weather and constantly wet conditions and will usually succumb to root rots in muggy climates.

Cedar-apple rust (Gymnosporangium juniperi-virginianae) produces hard stem galls in winter of up to 5 centimeters in width on susceptible junipers.

These are not seriously harmful to the juniper host, but in the spring the galls produce soft, gummy horns that release spores to infect apples and related plants in the rose family where it is a much more serious disease.

Due to the usually small size of their trunks they are not much utilized as timber except for making specialty products like "cedar" linings for closets or chests to repel moths.